In the integrated circuit (IC) industry, metal-oxide-semiconductor, (MOS) transistors have typically been formed utilizing polysilicon gate electrodes. Polysilicon material has been preferred for use as a MOS gate electrode due to its thermal resistive properties (i.e., polysilicon can better withstand subsequent high temperature processing). Polysilicon's robustness during high temperature processing allows polysilicon to be annealed at high temperatures along with source and drain regions. Furthermore, polysilicon's ability to block the ion implantation of doped atoms into a channel region is advantageous. Due to the ion implantation blocking potential of polysilicon, polysilicon allows for the easy formation of self-aligned source and drain structures after gate patterning is completed. However, polysilicon gate electrodes have certain disadvantages. For example, polysilicon gate electrodes are formed from semiconductor materials that suffer from higher resistivities than most metal materials. Therefore, polysilicon gate electrodes may operate at much slower speeds than gates made of metallic materials. To partially compensate for this higher resistance, polysilicon materials often require extensive and expensive silicide processing in order to increase their speed of operation to acceptable levels.
A need exists in the industry for a metal gate device which can replace a polysilicon gate device. However, metal gates cannot withstand the higher temperatures and oxidation ambients that can be withstood by conventional polysilicon gate electrodes. In efforts to avoid some of these concerns with polysilicon gate electrodes, a replacement damascene metal gate process has been created. A damascene gate process uses a disposable gate, and is formed with a source, drain, spacer, etch stops and anti-reflective coatings as in conventional processing. The disposable gate and dielectrics are etched away, exposing an original gate oxide. The disposable polysilicon gate is then replaced by a metal gate to achieve the lower resistivity provided by the metal material.
A design consideration in semiconductor technology is that of the work function, which is the amount of energy required to excite electrons across a threshold. Polysilicon gates on silicon substrates provide a work function that allows the gates to be adequately controlled. The use of metal, however, as the gate material on a silicon substrate may undesirably change the work function in comparison to polysilicon gates. This reduces the controllability of the gate.